This application claims the priority of German Application No. 100 01 888.2 filed Jan. 19, 2000, which is incorporated herein by reference.
This invention related to a method of coating a metal surface, particularly an inner surface of a weapon barrel by depositing thereon a chromium layer by electroplating for preventing erosions.
In weaponry, performance-enhanced ammunition causes significant erosions due to the high gas temperatures and flow velocities during firing, particularly in weapon barrels made of steel. Such erosions render the weapon barrel unusable before it reaches the end of its service life due to metal fatigue.
It is known to provide weapon barrels with a hard chromium layer for avoiding erosions of the above type. The hard chromium layer is deposited electrolytically on the inner face of the weapon barrel.
It is, among others, a disadvantage of such a known method that the electrolytically deposited hard chromium layers do not sufficiently withstand the effects of performance-enhanced ammunition. The resulting chromium breakouts often require a cost-intensive subsequent metal working.
Tests conducted by the applicants have shown that the chromium breakouts in the known hard chromium layers are caused to a large measure by the deposition-based (111) [uvw] texture and the bar-like microstructure of the chromium layer. These lead to a direction-dependent mechanical behavior of the chromium layers.
It has been found that in the conventional hard chrome deposition a distinction has to be made among three phases of crystallite growth: in a first phase a nucleation occurs, in a second phase the crystallite growth is in progress and in a third phase the crystallite growth is impeded and then stopped. The second phase results in a sharp (1-1-1) [uvw] texture of the chromium layer since such a texture layer has a higher growth velocity than other texture layers.
The slow-down of the crystallite growth or the growth stoppage is effected by impeding the growth of neighboring grains and/or by an increased defect rate in the growing process.
It is an object of the invention to provide an improved method of internally coating weapon barrels by means of a galvanically deposited hard chromium layer which does not lead to a significant, direction-dependent mechanical layer behavior, so that in use of the weapon barrel chromium breakouts are substantially prevented.
This object and others to become apparent as the specification progresses, are accomplished by the invention, according to which, briefly stated, the method of coating a metal surface, particularly an inner surface of a gun barrel, with a chromium layer, includes the following steps: electrolytically precipitating on the metal surface a plurality of partial chromium layers in a superposed relationship by electric current pulses equaling the number of the partial chromium layers; and selecting the duration of each pulse such that a crystallite growth of individual partial chromium layers is stopped prior to a natural termination thereof for obtaining a globular polytropic structure of the entire chromium layer.
While the chromium crystallites which are conventionally precipitated with a non-pulsed d.c. current, have a length of approximately 5-10 micron, when the precipitation is carried out according to the invention, the crystallites have a length of between approximately 0.2 and 2 micron. In this manner, by using a pulsed current, not only an isotropic texture is obtained, but also, a finer grain structure results which additionally leads to a better resistance of the chromium layer to stresses during service.
Apart from the fact that when using the method according to the invention, the obtained chromium layers are less sensitive to mechanical stresses than chromium layers produced conventionally, the method also has the advantage that by applying a plurality of chromium layers with polytrope structure, thicker, more stable chromium layers may be made than it has been possible with conventional methods.
By varying the current intensity and pulse duration the properties of the chromium layer precipitated on the inner surface of the weapon barrel may be varied between wide limits.